What are Long-Polling, Websockets, Server-Sent Events (SSE) and Comet?

Question

I have tried reading some articles, but I am not very clear on the concepts yet.

Would someone like to take a shot at explaining to me what these technologies are:

Answer 1

I have tried to make note about these and have collected and written examples from a java perspective.

HTTP for Java Developers

Reverse Ajax - Old style

Async Handling on server side

Reverse Ajax - New style

Server Sent Events

Putting it here for any java developer who is looking into the same subject.